1. Field of the Invention
This invention is directed to the production of hydrogen from hydrocarbons and more particularly to a process for deriving hydrogen from a hydrocarbon feed in which the starting material is subjected to stripping of its sulfur contents on a nickel sorbent and subsequently to reforming reaction on a steam reforming catalyst.
2. Description of the Prior Art
Hydrogen finds extensive application for instance as a starting material, a refining agent, a fuel and the like and has its source of supply from a variety of methods including electrolysis of water and steam reforming, partial oxidation, decomposition or dehydrogenation of hydrocarbons and alcohols. Hydrogen obtainable by electrolyzing water is highly pure, though costly, and hence applicable to special usage as in physical and chemical experiments. Hydrogen for industrial use as starting materials or product refiners is in most cases produced by steam-reforming or partially oxidizing relatively cheap, readily available materials and in such instance steam reforming of light hydrocarbons and alcohols is more often effected than partial oxidation of coals and heavy residual oils. Such light hydrocarbons and alcohols include for example methane, ethane, propane, butane and mixtures thereof and gases containing these hydrocarbons, light naphtha, heavy naphtha and methanol.
In general, hydrocarbons contain sulfur compounds which tend to adversely affect steam reforming catalysts. It is necessary therefore to remove sulfur contents from the hydrocarbon prior to reforming reaction as the catalyst is highly sensitive to those sulfur compounds.
A hydrogenative desulfurization process is known for removing sulfur compounds from hydrocarbons in the presence cf a hydrogen-containing gas with the use of a catalyst such as cobalt-molybdenum, nickel-molybdenum, nickel-cobalt-molybdenum, nickel-tungsten and the like and under elevated temperature and pressure conditions. Hydrogen sulfide thus formed is subsequently removed by adsorption on to a metal oxide such as zinc oxide, copper oxide, manganese oxide or iron oxide. Hydrogenative desulfurization differs in reaction conditions from steam reforming, entailing the disposition of reaction vessels, control devices, heat exchangers and other necessary facilities for use in desulfurization at a position upstream of and independently of the steam reformer. This leaves the problem that the whole apparatus gets bulky with tedious control.
With the prior method of adsorbing hydrogen sulfide with metal oxides, it has been difficult to remove sulfur compounds other than hydrogen sulfide to a level of concentration tolerable to reforming catalysts. These metal oxides are liable to invite a sharp decline in adsorptivity on contact with steam in reforming reaciton.
It has also been proposed that a nickel sorbent be used to remove traces of sulfur compounds from naphtha fractions in a naphtha reforming process for gasoline production, thereby maintaining a platinum catalyst fully activable in a subsequent reaction. In the chemical industry such sorbent has been utilized to adsorb a limited amount of sulfur contained in organic compounds as starting materials so that side reaction is alleviated during preparation of the compounds intended. The present inventors disclose certain adsorption methods in Japanese Patent Laid-Open Publication Nos. 63-35403, 1-188404, 1-188405 and 1-188406, which methods serve to remove sulfur contents from a kerosine fraction under specific conditions. However, the methods of Publication Nos. 63-35403, 1-188404 and 1-188405 have been found not still satisfactory because increased quantity of a nickel sorbent is required to achieve a 0.2 ppm level over prolonged length of time. With the method of Publication No. 1-188406, hydrogen-containing gas often outgoes together with carbon monoxide (CO) and carbon dioxide (CO.sub.2) from a desulfurization reactor with the results that CO and CO.sub.2 will react with H.sub.2 in the presence of a nickel sorbent and thus generate gaseous methane, leading to hazardous heat buildup.
As hydrogen-containing gas necessary for reforming reaction, part of reformed gas derivable from that reaction is usually recycled in which CO and CO.sub.2 are contained. This recycle gas is obtained substantially free from CO and CO.sub.2 only when passed through carbon monoxide modifiers and carbonate removers arranged downwardly of the steam reformer. Contact of a nickel solvent with a hydrocarbon at a temperature above 40.degree. C. at an inlet of the reformer has been experimentally proved susceptible to too much carbonaceous deposit for practical purposes.